In typical incandescent lamps a tungsten coil of a given length and wire diameter is used to radiate both visible light and IR radiation when an electrical current is passed through it.
The tungsten coil will sag over time, especially when the operating temperature exceeds 3000 C, as is known to happen in some demanding applications. It is known that the addition of potassium will reduce, but not eliminate, the coil sagging, as is shown from U.S. Pat. No. 2,012,825.
In the case of lamps used in a vertical, base-up position, that is, with the axis of the coil perpendicular to the ground, the sag will eventually cause a short circuit in the filament, which will lead to higher currents passing through the coil with a concomitant increase in coil temperature. The increase in temperature accelerates the coil sagging and causes a further compression of the turns of the coil. It has been suggested in U.S. Pat. No. 6,600,255 that this problem can somewhat be alleviated by using a coil having two distinct pitches with a wider pitch at the bottom of the coil.